Should throw TransactionOperationFailed on individual op failure

When an individual op fails, right now the txn lib throws a Couchbase.Transactions.Error.Internal.ErrorWrapperException which is marked as internal and I cannot use it to check the error and raise it up to the txn driver.

In Java, an external TransactionOperationFailed is thrown instead which can then be looked at.

See https://github.com/couchbaselabs/transactions-fit-performer/blob/master/performers/java/txn-performer-java/src/main/java/com/couchbase/twoway/TwoWayTransaction.java#L590 for reference.